In the manufacture of insulated wire, a resin coating is usually extruded around the wire in an extrusion head and the coated wire is thereafter quenched to set the coating. Many uses require uniform concentricity between such wire and coatings and it is therefore desirable to have a measuring means positioned downstream of the extruder head but upstream of the quenching bath to continuously indicate errors in concentricity. In this way, the extrusion apparatus may be adjusted to produce a uniform product. Because the coating is still somewhat soft as it leaves the extruder head, the measuring means should be of a non-contact type. One prior art example of a non-contact eccentricity gage may be found in U.S. Pat. No. 4,086,044.